As a solid electrolyte for a storage battery such as a Na—S battery or a Na-molten salt battery, a beta-alumina-based sintered compact having a high sodium ion conductivity (hereinafter referred to simply as “ionic conductivity”) has been used. The beta-alumina is classified into two types of crystalline form, namely β alumina of which the chemical composition is represented by Na2O.xAl2O3 (x=9 to 11) and β″ alumina of which the chemical composition is represented by Na2O.xAl2O3 (x=5 to 7). Hereinafter, in the present specification, “beta-alumina” is used as a general term for β alumina and β″ alumina. Among them, since β″ alumina has a higher content of sodium ions in the crystalline structure and a relatively high ionic conductivity, β″ alumina is used in applications for a solid electrolyte in many cases. On the other hand, it is known that since β″ alumina is a metastable substance, its crystalline structure is hardly maintained.
Irrespective of its crystalline form, beta-alumina is hardly sinterable, and therefore, the firing temperature is required to be high at a level of at least 1,600° C. in order to obtain a dense sintered compact having a high density. Since the beta-alumina contains as the essential component, Na2O which easily volatilizes at a high temperature, it is desired to lower the firing temperature.
Heretofore, as a method to lower the firing temperature in firing beta-alumina, a liquid phase sintering has been proposed. In the liquid phase sintering, a sintering agent which becomes a liquid phase at a relatively low temperature is added to carry out firing. If the sintering agent is not properly selected, anomalous grain growth results among particles one another, and densification of a sintered compact is rather impaired, and the mechanical strength tends to deteriorate. Further, after firing, the sintering agent may sometimes remain in the sintered compact as a crystalline phase, whereby the durability, etc. of the sintered compact tend to be impaired.
Patent Document 1 proposes a process for obtaining a dense sintered compact, which comprises adding Li2O as a stabilizer for β″ alumina in a weight ratio of from 0.5 to 0.75 wt % per the entire sintered compact, whereby the firing temperature in a liquid phase sintering can be lowered to 1,450° C. However, the sintered compact in which Li2O is used as a stabilizer is insufficient in durability, if used for a long period of time.
Patent Document 2 proposes a process which comprises adding Bi2O3, CuO or PbO as a sintering agent to lower the firing temperature of a beta-alumina to from 1,300 to 1,400° C., whereby the volatilization of Na2O can be suppressed. However, in such a process, the maximum relative density of the sintered compact is 94% at most, and it is not possible to obtain a dense sintered compact having a high mechanical strength.